Ribbon Crystal String with Extruded Refractory Material

ABSTRACT

A method of making string for string ribbon crystal provides a substrate having an outer surface, and extrudes refractory material over the substrate. The refractory material substantially covers the outer surface of the substrate. The method then cures the refractory material.

PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority from provisional U.S. patentapplication No. 60/969,263, filed Aug. 31, 2007, entitled, “STRINGRIBBON CRYSTAL AND STRING WITH IMPROVED EFFICIENCY,” assigned attorneydocket number 3253/106, and naming Christine Richardson, LawrenceFelton, Richard Wallace, and Scott Reitsma as inventors, the disclosureof which is incorporated herein, in its entirety, by reference.

This patent application also is related to the following copending,co-owned patent applications, filed on even date herewith, claiming thesame priority as noted above and incorporated herein, in theirentireties, by reference:

Attorney Docket Number 3253/172, entitled, “REDUCED WETTING STRING FORRIBBON CRYSTAL,” and

Attorney Docket Number 3253/173, entitled, “RIBBON CRYSTAL STRING FORINCREASING WAFER YIELD.”

FIELD OF THE INVENTION

The invention generally relates to string ribbon crystals and, moreparticularly, the invention also relates to string used to form stringribbon crystals.

BACKGROUND OF THE INVENTION

String ribbon crystals, such as those discussed in U.S. Pat. No.4,689,109 (issued in 1987 and naming Emanuel M. Sachs as the soleinventor), can form the basis of a variety of electronic devices. Forexample, Evergreen Solar, Inc. of Marlborough, Mass. forms solar cellsfrom conventional string ribbon crystals.

As discussed in greater detail in the noted patent, conventionalprocesses form string ribbon crystals by passing two or more stringsthrough molten silicon. The composition and nature of the string canhave a significant impact on the efficiency and, in some instances, thecost of the ultimately formed string ribbon crystal.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a method of makingstring for string ribbon crystal provides a substrate having an outersurface, and extrudes refractory material over the substrate. Therefractory material substantially covers the outer surface of thesubstrate. The method then cures the refractory material.

For example, the substrate may be formed from a carbon filament or atow, while the extruded refractory material may include silicon carbide.The method also may form an exterior reduced wetting layer radiallyoutward of the refractory material. In some embodiments, the substrateand refractory material form a generally elongated cross-sectionalshape, and/or are generally concentric.

In other embodiments of the invention, a string for forming a ribboncrystal has a substrate, and an extruded refractory material layersubstantially covering the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Those skilled in the art should more fully appreciate advantages ofvarious embodiments of the invention from the following “Description ofIllustrative Embodiments,” discussed with reference to the drawingssummarized immediately below.

FIG. 1 schematically shows a string ribbon crystal that may be formedfrom strings configured in accordance with illustrative embodiments ofthe invention.

FIG. 2 schematically shows an illustrative furnace used to form stringribbon crystals.

FIG. 3 schematically shows a cross-sectional view of a portion of aprior art ribbon crystal with a prior art string.

FIG. 4A schematically shows a string formed in accordance withillustrative embodiments of the invention.

FIG. 4B schematically shows eight cross-sectional views of the string ofFIG. 4A along line B-B in accordance with various embodiment of theinvention.

FIG. 5 shows an illustrative process of forming a string ribbon crystalusing strings configured in accordance with illustrative embodiments ofthe invention.

FIGS. 6A, 6B, and 6C schematically show cross-sectional views of ribboncrystals in accordance with an embodiment using strings with anelongated cross-section.

FIGS. 7A and 7B schematically show cross-sectional views of ribboncrystals with multiple strings used to perform the function of a singlestring.

FIGS. 8A and 8B schematically show a ribbon crystal with a string havinga generally concave cross-sectional shape.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments extrude a refractory material over acore/substrate to form string used to grow ribbon crystals. This processbeneficially avoids use of complex prior art processes that requirehazardous chemicals (e.g., CVD processes). Details of variousembodiments are discussed below.

FIG. 1 schematically shows a string ribbon crystal 10 configured inaccordance illustrative embodiments of the invention. In a mannersimilar to other ribbon crystals, this ribbon crystal 10 has a generallyrectangular shape and a relatively large surface area on its front andback faces. For example, the ribbon crystal 10 may have a width of about3 inches, and a length of about 6 inches. As known by those skilled inthe art, the length can vary significantly. For example, in some knownprocesses, the length depends upon a furnace operator's discretion as towhere to cut the ribbon crystal 10 as it grows. In addition, the widthcan vary depending upon the separation of its two strings 12 (see FIG.2) forming the ribbon crystal width boundaries. Accordingly, discussionof specific lengths and widths are illustrative and not intended tolimit various embodiments the invention.

The thickness of the ribbon crystal 10 may vary and be very smallrelative to its length and width dimensions. For example, the stringribbon crystal 10 may have a thickness ranging from about 60 microns toabout 320 microns across its width. Despite this varying thickness, thestring ribbon crystal 10 may be considered to have an average thicknessacross its length and/or width.

The ribbon crystal 10 may be formed from any of a wide variety ofmaterials (often referred to generally as “ribbon material” or “crystalmaterial”), depending upon the application. For example, when grown fora photovoltaic application, the ribbon crystal 10 may be formed from asingle element, such as silicon, or a compound, such as a silicon-basedmaterial (e.g., silicon germanium). Other illustrative ribbon materialsmay include gallium arsenide, or indium phosphide. The ribbon materialmay be any of a variety of crystal types, such as multi-crystalline,single crystalline, polycrystalline, microcrystalline orsemi-crystalline.

As known by those skilled in the art, the ribbon crystal 10 is formedfrom a pair of strings 12 generally embedded/encapsulated by the ribbonmaterial. For simplicity, the ribbon crystal 10 is discussed as beingformed from polysilicon ribbon material. It nevertheless should bereiterated that discussion of polysilicon is not intended to limit allembodiments.

Illustrative embodiments grow the ribbon crystal 10 in a ribbon crystalgrowth furnace 14, such as that shown in FIG. 2. More specifically, FIG.2 schematically shows a silicon ribbon crystal growth furnace 14 thatmay be used to form the string ribbon crystal 10 in accordance withillustrative embodiments of the invention. The furnace 14 has, amongother things, a housing 16 forming a sealed interior that issubstantially free of oxygen (to prevent combustion). Instead of oxygen,the interior has some concentration of another gas, such as argon, or acombination of gasses. The housing interior also contains, among otherthings, a crucible 18 and other components for substantiallysimultaneously growing four silicon ribbon crystals 10. A feed inlet 20in the housing 16 provides a means for directing silicon feedstock tothe interior crucible 18, while an optional window 22 permits inspectionof the interior components.

As shown, the crucible 18, which is supported on an interior platformwithin the housing 16, has a substantially flat top surface. Thisembodiment of the crucible 18 has an elongated shape with a region forgrowing silicon ribbon crystals 10 in a side-by-side arrangement alongits length. In illustrative embodiments, the crucible 18 is formed fromgraphite and resistively heated to a temperature capable of maintainingsilicon above its melting point. To improve results, the crucible 18 hasa length that is much greater than its width. For example, the length ofthe crucible 18 may be three or more times greater than its width. Ofcourse, in some embodiments, the crucible 18 is not elongated in thismanner. For example, the crucible 18 may have a somewhat square shape,or a nonrectangular shape.

As shown in FIG. 2 and discussed in greater detail below, the furnace 14has a plurality of holes 24 (shown in phantom) for receiving string 12.Specifically, the furnace 14 of FIG. 2 has eight string holes 24 forreceiving four pairs of strings 12. Each pair of strings 12 passesthrough molten silicon in the crucible 18 to form a single ribboncrystal 10.

Many conventional ribbon crystal growth processes form ribbon crystalswith a thin neck portion near the string. More specifically, FIG. 3schematically shows a cross-sectional view of a portion of a prior artribbon crystal 10P having a prior art string 12P. This prior art ribboncrystal 10P has a thin neck portion 36 between the string 12P and awider portion 38 of the ribbon crystal 10. If the neck portion 36 is toothin, then the ribbon crystal 10P may be very fragile and more prone tobreaking, thus leading to yield losses. For example, if the coefficientof thermal expansion differential between the string 12 and ribbonmaterial forming the ribbon crystal 10P (e.g., polysilicon) issufficiently large, the ribbon crystal 10P may be more prone to breakingat the neck portion 36.

To increase the neck thickness, those skilled in the art have addedequipment to the ribbon growth process. For example, one such solutionadds gas jets (not shown) to the furnace 14. These gas jets directrelatively cool gas streams toward the neck portion 36, thus decreasingthe temperature in that area to increase neck thickness. Other solutionsinvolve adding specialized meniscus shapers.

Rather than use such additional external measures, illustrativeembodiments of the invention engineer the cross-sectional dimension ofthe string 12 in a prescribed manner. The string 12 then is positionedwithin the crystal growth furnace 14 in a manner that increases the sizeof the neck portion 36 of the growing ribbon crystal 10. For example,the resulting ribbon crystal 10 with an average thickness of about 190microns may have a neck portion 36 with a minimum thickness of about 60microns, which may suffice in certain applications. This innovationconsequently should reduce yield loss, thus reducing production costs.

FIG. 4A schematically shows a string 12 that may be formed in accordancewith illustrative embodiments of the invention. Although this figureappears to show a generally convex or rounded cross-section, it shouldbe considered merely schematic and not representative of any specificcross-sectional shape. To that end, FIG. 4B schematically shows eightdifferent possible cross-sectional views of the string 12 of FIG. 4Aalong cross-line B-B in accordance with a number of differentembodiments of the invention. For example, some of the shapes aregenerally elongated, such as the irregular shape of string one, therectangular shape of string two, and the somewhat elliptical shape ofstring three.

Whether or not they are elongated, the various strings 12 may becategorized as being either generally concave or generally convex. Asused herein, a cross-sectional shape is generally concave when anyportion of its perimeter forms at least one non-negligible concavity.Thus, string one is considered to be generally concave despite its otherconvex portions. Conversely, a cross-sectional shape is considered to begenerally convex when its perimeter forms no non-negligible concavities.Thus, string two and string three of FIG. 4B a generally convex.

FIG. 4B shows a number of other cross-sectional string shapes that aregenerally concave. In fact, some may be considered elongated andconcave. For example, string four is generally “C” shaped, concave, andelongated, while string five is generally cross shaped, concave, but notelongated. The shape of string five (cross shaped) is not elongatedbecause it is generally symmetrical—both the horizontal and verticalportions of the cross are about the same size. Depending upon its actualdimensions, string eight, which is generally “T” shaped, may or may notbe considered elongated. For example, if the portion of the “T” shapeextending downwardly is longer than its horizontal portion, then stringeight may be considered elongated. In either case, string eight isconsidered to be generally concave.

Some embodiments use plural strings 12 to form one edge of a ribboncrystal 10. Strings six and seven show two such embodiments.Specifically, string six shows one embodiment where the individualstrings 12 physically contact each other in the final ribbon crystal 10,while string seven shows another embodiment where the individual strings12 are spaced from each other in the final ribbon crystal 10. It shouldbe noted that embodiments using plural strings 12 may use more than twostrings 12. In addition, individual strings 12 of this plural stringembodiment may have the same or different cross-sectional shapes (e.g.,a first elliptically shaped string 12 and another cross or circularshaped string 12).

The specific shapes of FIG. 4B merely are examples of a variety ofdifferent cross-sectional string shapes. For example, some embodimentsuse strings that have a generally circular cross-sectional shape.Accordingly, those skilled in the art should understand that otherstring shapes fall within the scope of various embodiments.

FIG. 5 shows an illustrative process of forming a string ribbon crystal10 with strings 12 configured in accordance with illustrativeembodiments of the invention. For simplicity, this process is discussedwith reference to string two of FIG. 4B only—because string two is theonly string 12 in that figure explicitly showing various string layersdiscussed in this process. It nevertheless should be noted that thediscussed principles apply to strings 12 having other cross-sectionalshapes, or other strings formed by other processes.

The process begins at step 500 by forming a core/substrate 28, whichacts as a substrate to receive a refractory material layer. As discussedin greater detail in co-pending US patent application having attorneydocket number 3253/172 and entitled, “REDUCED WETTING STRING FOR RIBBONCRYSTAL,” (which is incorporated by reference above), the core 28 can beformed from carbon by conventional extruding processes. In otherembodiments, however, the core 28 may be a wire, filament, or pluralityof small conductive fibers wound together as a tow. For example,post-fabrication processes could form a monofilament through a knownfabrication process, such as oxidation, carbonization, or infiltration.

The core 28 may have the desired cross-sectional shape. For example, asshown in FIG. 4B, the core 28 of string two is generally rectangular.Alternatively, the core 28 may have a different cross-sectional shape,while refractory material application equipment may be speciallyconfigured to form the desired cross-sectional shape. For example, theextrusion equipment may be specially configured to form thecross-sectional shape from a core material having a prespecifiedcross-sectional shape that is the same as or different than that of thefinal cross-sectional string shape.

After forming the core 28, the process forms a first coating/layer,which acts as the above noted refractory material layer 30 (step 502).Among other things, the first coating 30 may include silicon carbide,tungsten, or a combination of silicon carbide and tungsten. Conventionalwisdom dictates that this outer surface 30 should be very smooth tominimize nucleations that may occur when it contacts molten ribbonmaterial within the furnace 24. Fewer nucleations desirably shouldproduce fewer grains and thus, fewer grain boundaries. Consequently,such strings 12 should be more electrically efficient than those withmore grains and more grain boundaries.

To those ends, one commonly used prior art process known to theinventors uses chemical vapor deposition (i.e., “CVD”) to form therefractory material layer 30. Accordingly, such prior art strings shouldhave smoother outer surfaces and thus, produce fewer grains and grainboundaries. Undesirably, however, such a process is complex and useshazardous chemicals.

Illustrative embodiments solve these problems. Specifically, to avoidthe use of such complex machinery and hazardous chemicals of a CVDprocess (or other similar process), illustrative embodiments extrude therefractory material directly onto the core/substrate 28, thus coveringsubstantially the entire outer (circumferential) surface of the core 28.This is contrary to prior art teachings, however, because it is expectedto yield a less smooth surface. The inventors nevertheless anticipatethat such a string can produce satisfactory results in a much lesscostly manner and with fewer safety risks.

Formation of the extruded refractory material layer 30 may involve,among other things, a pulltrusion process, or both spinning of arefractory material with a polymer component, which subsequently isbaked off. Processes may use at least one component of carbon, silicon,silicon carbide, silicon nitride, aluminum, mullite, silicon dioxide, BNparticles, or fibers mixed with a polymer binder, coupled withextrusion/pulltrusion. This also may involve bicomponent extrusion of acore 28 with at least one silicon carbide, carbon, silicon, and a sheathwith a least one of oxide, mullite, carbon, and/or silicon carbide.Accordingly, as noted above, the core 28 effectively acts as a substratefor supporting the refractory material layer 30. For example, therefractory material layer 30 may be, or may not be, generally concentricwith the core 28. After it is extruded onto the core 28, the refractorymaterial layer 30 is allowed to harden/cure for a sufficient amount oftime.

As discussed below, some embodiments form one or more layers radiallyoutward of the refractory material layer 30. Such layers can besmoother, or take on a roughness that is similar to that of this layer30.

This step thus forms what is considered to be a base string portion 26.At this point in the process, the base string portion 26 has a combinedcoefficient of thermal expansion that preferably generally matches thecoefficient of thermal expansion of the ribbon material. Specifically,the thermal expansion characteristics of the string 12 should besufficiently well matched to the ribbon material so that excessivestress does not develop at the interface. Stress is considered excessiveif the string 12 exhibits a tendency to separate from the ribbon duringreasonable subsequent ribbon crystal handling and processing steps, orif the string 12 exhibits a tendency to curl outwardly or inwardly fromthe ribbon crystal edge. In other embodiments, however, the coefficientof thermal expansion of the base string portion 26 does not generallymatch that of the ribbon material.

As noted above, some embodiments of the invention may have one or moreadditional layers, depending upon the application. For example, asdiscussed in greater detail in the above noted incorporated patentapplication having attorney docket number 3253/172, the string 12 mayhave a non-wetting/reduced wetting layer 32 to increase the grain sizeof the ribbon material. In that case, the process continues to step 504,which forms an exposed non-wetting/reduced layer 32 on the base stringportion 26. In applications sensitive to coefficient of thermalexpansion differences, this layer 32 preferably is very thin so that ithas a negligible impact on the overall string coefficient of thermalexpansion. For example, the reduced wetting layer 32 should be muchthinner than that of the refractory material layer 30.

In embodiments using this non-wetting layer 32, the contact angle withthe ribbon material of its exterior surface should be carefullycontrolled to cause the molten ribbon material to adhere toit—otherwise, the process cannot form the ribbon crystal 10. Inapplications using molten polysilicon, for example, it is anticipatedthat contact angles with silicon of between about 15 and 120° degreesshould produce satisfactory results. Such angles of greater than 25degrees may produce better results.

Among other ways, the non-wetting layer 32 may be formed by CVDprocesses, dip coating or other methods. For example, the base stringportion 26 may be CVD coated by applying electrical contacts in adeposition chamber while it is being fed through the chamber—thusheating the base string portion 26 itself. Alternatively, the basestring portion 26 may be heated by induction heating through thechamber.

Related techniques for implementing this step include:

-   -   a sol gel dip for silica or alumina oxide or silicon oxycarbide        either at the end of a CVD furnace or during rewind,    -   a CVD nonwetting coating deposited by heating quartz from the        outside and induction heating the base string portion 26,    -   spray-on deposition with a polymer binder that subsequently        would be burned off,    -   shaking particles onto a base string portion 26 or tow and then        baking the into the base string portion 26 or tow, and    -   coating with base string portion 26 with refractory slurry        (e.g., silicon carbide/silicon dioxide) or liquid and then        burning off residual.

The string 12 also may have a handling layer 34 radially outward of therefractory material layer 30 to maintain the integrity of the basestring portion 26. To that end, if included, the handling layer 34provides a small compressive stress to the base string portion 26, thusimproving robustness to the overall string 12. Accordingly, if the basestring portion 26 develops a crack, the compressive stress of thehandling layer 34 should reduce the likelihood that the string 12 willbreak. Among other things, the handling layer 34 may be a thin layer ofcarbon (e.g., one or two microns thick for strings 12 having generallyknown sizes).

Accordingly, prior to performing step 504, some embodiments may form ahandling layer 34 that is separate from the produced nonwetting layer 32(e.g., see string two of FIG. 4B). Thus, in such an embodiment, thenonwetting layer 32 substantially covers the handling layer 34. Morespecifically, the nonwetting layer 32 covers the outer, circumferentialsurface of the handling layer 34. Some embodiments, however, mayintegrate the non-wetting layer 32 into the handling layer 34.

It then is determined at step 506 if the coated string 12 has filamentsextending through the nonwetting layer 32 (such filaments are referredto herein as “whiskers”). This can occur, for example, when a tow offilaments forms the core 28. If the coated string 12 has whiskers, thenthe process shaves them off at step 508. The process then may loop backto step 504, which re-applies the nonwetting layer 32.

Alternatively, if the string 12 has no whiskers, the process continuesto step 510, which provides the string 12 to the furnace 14 as shown inFIG. 2. To that end, some embodiments provide a single string 12 foreach ribbon crystal edge, or multiple strings 12 for each ribbon crystaledge (e.g., strings six and seven of FIG. 6B). The term “string,” unlessexplicitly modified to the contrary (e.g., by the words “single” or“multiple”), when mentioned with reference to forming a boundary/widthof a ribbon crystal 10, generally means one or more strings.

Rather than using the methods above for forming the string 12, someembodiments machine or bore a concavity into a rounded or otherotherwise generally convex string 12. Accordingly, the string 12 may beformed by other methods.

Illustrative embodiments orient the strings 12 in the furnace 14 in amanner that increases the thickness of the ribbon crystal neck portion36. For example, FIGS. 6A-6C schematically show cross-sectional views ofthree ribbon crystals 10 with strings 12 having elongated, generallyelliptical, generally convex cross-sectional shapes. To increase thethickness of the neck portion 36, these embodiments orient theirrespective generally longitudinal axes 42 so that they diverge with thewidth dimension of their respective ribbon crystals 10. In other words,to diverge, the longitudinal axis 42 is not parallel with the widthdimension—instead, the longitudinal axis 42 and width dimensionintersect.

More specifically, the cross-section of each string 12 has a largestdimension, each of which is shown as double-head arrows in FIGS. 6A-6C.For reference purposes, the longitudinal axis 42 of each of theseelongated cross-sectional shapes thus is considered to be co-linear withthe largest dimension. For example, FIG. 6A orients the longitudinalaxis 42 substantially perpendicular to the width dimension, while FIG.6C orients the longitudinal axis 42 to form a shallow angle with thewidth dimension. FIG. 6B orients the longitudinal axis 42 between theextremes of FIGS. 6A and 6C.

It should be noted that orientations other than those shown in FIGS.6A-6C also should provide satisfactory results. For example, orientingthe longitudinal axis 42 in a manner so that is rotated about 90 degrees(either clockwise or counterclockwise) from the angle shown in FIG. 6Balso should increase neck size.

FIGS. 8A and 8B schematically show two ribbon crystals 10 with strings12 having a generally concave cross-sectional shape. As shown, thestrings 12 are oriented so that their concavities either are orientedcompletely toward or completely away from the wafer width (i.e., in theX-direction). In particular, the concavity is generally symmetricallyoriented, e.g., the concavity forms a mirror image above and below theX-axis. Significant rotation from these orientations (either clockwiseor counterclockwise), however, may impact the meniscus shape to impedeappropriate crystal growth. Those in the art can apply this concept to astring 12 having multiple concavities or concavities on opposing sidesof the cross-sectional shape (e.g., a cross-shape).

At this point, for each ribbon crystal 10 being grown, the processpasses two strings 12 (together forming the ultimate ribbon crystalwidth) through the furnace 14 and crucible 18, thus forming the stringribbon crystal 10 (step 512).

Accordingly, illustrative embodiments of the invention extrude therefractory material layer 30 on the core 28, thus avoiding problemsassociated with prior art deposition techniques and reducing productioncosts.

Although the above discussion discloses various exemplary embodiments ofthe invention, it should be apparent that those skilled in the art canmake various modifications that will achieve some of the advantages ofthe invention without departing from the true scope of the invention.

1. A method of making string for string ribbon crystal, the methodcomprising: providing a substrate having an outer surface; extrudingrefractory material over the substrate, the refractory materialsubstantially covering the outer surface of the substrate; curing therefractory material.
 2. The method as defined by claim 1 wherein thesubstrate comprises a carbon filament.
 3. The method as defined by claim1 wherein the refractory material comprises silicon carbide.
 4. Themethod as defined by claim 1 wherein the substrate comprises a tow. 5.The method as defined by claim 1 further comprising forming an exteriorreduced wetting layer radially outward of the refractory material. 6.The method as defined by claim 1 wherein the substrate and refractorymaterial form a generally elongated cross-sectional shape.
 7. The methodas defined by claim 1 wherein the substrate and refractory material aregenerally concentric.
 8. A string for forming a ribbon crystal, thestring comprising: a substrate having an outer surface; and an extrudedrefractory material layer substantially covering the outer surface ofthe substrate.
 9. The string as defined by claim 8 wherein the substratecomprises a carbon filament.
 10. The string as defined by claim 8wherein the refractory material comprises silicon carbide.
 11. Thestring as defined by claim 8 wherein the substrate comprises a tow. 12.The string as defined by claim 8 further comprising an exterior reducedwetting layer radially outward of the refractory material.
 13. Thestring as defined by claim 8 wherein the substrate and refractorymaterial form a generally elongated cross-sectional shape.
 14. Thestring as defined by claim 8 wherein the substrate and refractorymaterial are generally concentric.
 15. The string as defined by claim 8further comprising a handling layer radially outward of the refractorymaterial.
 16. A string for forming a ribbon crystal, the stringcomprising: a substrate; and extruded refractory means substantiallycovering the substrate.
 17. The string as defined by claim 16 whereinthe extruded refractory means comprises a refractory material.
 18. Thestring as defined by claim 16 wherein the substrate comprises a carbonfilament.
 19. The string as defined by claim 16 wherein the extrudedrefractory means comprises silicon carbide.
 20. The string as defined byclaim 16 wherein the substrate comprises a tow.
 21. The string asdefined by claim 16 further comprising an exterior reduced wetting layerradially outward of the extruded refractory means.